Review the item:
During the fall semester, parking on campus is very difficult, students
circle the lots like vultures,
waiting for an empty space, and most students soon realize that taking
a 7 a.m. class, despite the effort it takes to get up that early, at least
cures their parking woes.
The comma between vultures and waiting is completely correct. Students circle the lots like vultures is a main clause. Waiting for an empty space is a present participle phrase. Present participle phrases will begin with an ing word like waiting.
When a present participle phrase follows a main clause, you will often need a comma between them.